Visual excitation is initiated by the absorption of a photon by the 11-cis retinal chromophore in the visual pigment, rhodopsin. This event is followed by a sequence of conformational changes in the protein and by the cis direction trans isomerization of the chromophore. The long term objective of this proposal is to determine in detail the molecular mechanism for these events through the use of resonance Raman spectroscopy. This objective will be approached in the following ways: (1) The Raman spectra of rhodopsin and its primary photoproduct, bathorhodopsin, will be obtained using pigments that have been regenerated with 2H and 13C isotopic derivatives of retinal. These data will be used to assign the vibrations in these spectra. (2) The Raman and IR spectra of isotopically labeled retinal model compounds will be obtained to provide vibrational assignments for the uncomplexed chromophores. (3) A ground state modified-Urey-Bradley force field will be refined following the Wilson FG method to fit the observed vibrational data for the retinal model compound and for the pigments. The vibrational frequencies, coupling constants, and Raman intensities for the pigment bound chromophore will then be analyzed to determine the structure of the retinal chromophore in rhodopsin and bathorhodopsin. These data will be used to develop models for the chromophore-protein interactions that make these pigments such sensitive photodetectors. (4) Models for the structure of rhodopsin and bathorhodopsin which involve charge perturbation of the chromophore by the protein will be evaluated with 13C NMR experiments and CNDO calculations. (5) Resonance Raman microscopy will be used to perform analogous studies on the primary visual photochemistry in rod and cone pigments which have a wide range of absorption maxima. This will test the generality of the models developed for bovine pigments in (1) - (4).